This chapter provides a comprehensive overview of nanofluids and hybrid nanofluids, emphasizing their exceptional thermal, electrical, and optical properties. The discussion delves into the applications of these fluids in heat transfer, mass transfer, and fluid flow, highlighting their potential to revolutionize various industries. The chapter also explores the synergistic effects of hybrid nanofluids, which enable multi-functional applications and offer tremendous opportunities for innovation. Furthermore, the chapter addresses the key challenges, standardization, and scalability issues associated with nanofluids and hybrid nanofluids. The conclusion outlines future research directions, emerging opportunities, and potential breakthroughs in fields such as electronics, aerospace, healthcare, energy, and biomedicine.